In lymphocytes, signals from antigen, cytokine, and innate immune receptors cause immediate, transient activation of NF-:B through the canonical pathway by triggering IkB degradation. A second, non-canonical or alternative NF-kB pathway has been described downstream of some members of the TNFR family. Slow, sustained activation of NF-kB through this IkB-independent, NIK-dependent non-canonical pathway governs the formation of secondary lymphoid organs (downstream of LT2R) and determines the fate of B cells (downstream of BAFFR). Almost nothing is known of the role of the non-canonical NF-kB pathway in T cell function, although T cells express a number of costimulatory TNFR family members, including OX40, 4-1BB, CD27, GITR, CD30, and HVEM, that have been shown to activate the non-canonical pathway in transfected cell lines, and that are known to provide essential signals for function and survival of activated T cells after antigen recognition. The working hypothesis of this proposal is that sustained activation of NF-kB through the non-canonical pathway downstream of these TNFR family members is necessary for induction and maintenance of expression of the cytokines, cytokine receptors, and anti-apoptotic proteins that allow T cells to survive and function as differentiated effector and memory cells. The proposed experiments are designed to determine whether the non-canonical pathway is activated downstream of the costimulatory TNFR family members in T cells, whether it is necessary for the costimulatory activity of TNFR family members in vivo, and whether independent activation of that pathway mimics some of the costimulatory activities of the TNFR family members, including blocking the function of regulatory T cells.